[#867] Improve video encoders timestamp handling

Details
This patch improves timestamp handling on video encoder elements.We need this
because buffer returned from circular does not contain correct timestamp data.
In this implementation, we are using glist object to maintain upstream buffer
meta data and later used in encode_thread to get the timestamp. It works okay
with encoder because in most of case we receive full-frame during encoding
process.

Thanks
Brijesh Singh

Details
This patch improves timestamp handling on video encoder elements.We need this
because buffer returned from circular does not contain correct timestamp data.
In this implementation, we are using glist object to maintain upstream buffer
meta data and later used in encode_thread to get the timestamp. It works okay
with encoder because in most of case we receive full-frame during encoding
process.